Such bulges are common around volcanoes, and most stop growing after a few years, scientists said.

The U.S. Geological Survey says the South Sister uplift probably started in late 1997, pushed by magma pooling four miles below the surface. The ground has risen about 9 inches in all. The bulge is about 10 miles in diameter.

Early on, it rose about an inch or two per year. Now, it's moving up at about 3?10 of an inch per year.

"The rate now is low enough now that it is almost undetectable," said Dan Dzurisin, a research geologist with the U.S. Geological Survey at the Cascade Volcano Observatory in Vancouver, Wash.

A federal scientist studying satellite radar images noticed the bulge in 2000. The Geological Survey set up an array of sensors tied into the Global Positioning System and collected years' worth of data. The data shows the diminishing growth.

"We know now, in hindsight, that the rate has been declining since the very beginning," Dzurisin said.

South Sister rises 10,358 feet. Scientists consider it an active volcano and say it last erupted 2,000 years ago.

The heart of the bulge is about three miles west of South Sister and actually closer to a lesser-known peak, The Husband, whose summit is 7,524 feet.

The bulge isn't evident to people on the ground.

"It is a very, very subtle, very, very broad uplift of the surface," Dzurisin said.